We make pretty extensive use of Tab Panels in our application. We are trying to migrate to Ext 4.0, but the performance of the Tab Panels is too slow to be usable. It is bearable in Chrome, FF4 and IE9 (still noticeably slower in all browsers than 3.4 though), but most of our customers are still using IE7 and IE8 where the new Tab Panels are extremely slow.

I timed both of these examples on IE8, loading once to cache everything and then timing at the push of the refresh button:

Ext 3.4, load time: 1.9s
Ext 3.4, tab switch: instant

Ext 4.0.2, load time: 8s
Ext 4.0.2, tab switch: 1.5s

Here is a minimal example that illustrates the problem.

Ext 4.0.2:

Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>Tab Panel Test</title>
    
    <link rel="stylesheet" type="text/css" href="ext4.0/resources/css/ext-all.css" />
    <script type="text/javascript" src="ext4.0/ext-all.js"></script>

    
    <script type="text/javascript">
    //<![CDATA[
    Ext.onReady(function() {
        
        var itemNumber = 0;
        var createItems = function() {
            var items = [];
            for (var i = 0; i < 5; i++) {
                items.push({
                    xtype: 'textfield',
                    fieldLabel: 'Text ' + itemNumber++
                });
            }
            return items;
        };
        
        var panels = [];
        
        for (var i = 0; i < 100; i++) {
            panels.push({
                title: 'Panel ' + i,
                items: createItems()
            });
        }

        var panel = new Ext.tab.Panel({
            title: 'Sectioned Panel Test',
            width: 900,
            renderTo: Ext.getBody(),
            items: panels
        });
    });
    
    //]]>
    </script>
    
</head>
<body>
</body>
</html>
Ext 3.4:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>Sectioned Panel Test</title>

    <link rel="stylesheet" type="text/css" href="ext3.4/css/ext-all.css" />
 
    <script type="text/javascript" src="jquery/jquery-1.4.2.min.js"></script>
    <script type="text/javascript" src="ext3.4/adapter/jquery/ext-jquery-adapter.js"></script>
    <script type="text/javascript" src="ext3.4/ext-all.js"></script>
    
    
    <script type="text/javascript">
    //<![CDATA[
    Ext.onReady(function() {
        
        var itemNumber = 0;
        var createItems = function() {
            var items = [];
            for (var i = 0; i < 5; i++) {
                items.push({
                    xtype: 'textfield',
                    fieldLabel: 'Text ' + itemNumber++
                });
            }
            return items;
        };
        
        var panels = [];
        
        for (var i = 0; i < 100; i++) {
            panels.push({
                title: 'Panel ' + i,
                layout: 'form',
                autoHeight: true,
                items: createItems()
            });
        }

        var panel = new Ext.TabPanel({
            title: 'Sectioned Panel Test',
            width: 900,
            renderTo: Ext.getBody(),
            items: panels
        });
    });
    
    //]]>
    </script>
    
</head>
<body>
</body>
</html>
Of course our actual pages are a lot more complicated than these examples (less tabs, ~20, but a lot of form fields, grids and inter-dependencies between tabs), but this is a pretty good representation of what is happening on our pages.